Buck Regulator Architectures

4.5 Current/Emulated Current Mode Buck Regulators

CURRENT MODE

2

Current-Mode Buck Regulator

3

VIN

RC

CC

COUT

SlopeComp

RESR

RLOAD

L

RFB2

RFB1

VOUT

VBG

+

+

VFBVC

+

-+-

Output Filter

Error Amplifier

Modulator

Current-Mode Buck-Regulator Architecture

4

+

-

PWM Comparator

VC

Corrective Ramp

VINModulator and Power Stage

T

Sn

Se

+ +

RS

VOUT Reference

A(s)+

-

Feedback , Error Amplifier,and Compensation

RL

RC(ESR)

VOUTL

C

+

-

Current SenseAmplifier

A i

T

DT

DT

Integrated or external

Advantages and Disadvantages• Advantages

– Power plant gain offers a single-pole roll-off– Line rejection– Cycle-by-cycle current limiting protection– Current sharing

• Disadvantages– Noise– Minimum ON-time– Sense resistor

5

CMC Sub-Harmonic Oscillation• Current mode

controlled power converters operating at duty cycles >50% are prone to sub-harmonic oscillation

• Disturbances in peak rising current ( I) increase at the end of the cycle

6

D =0.6

D = 0.33

Slope Compensation

7

  

cm Internal Slope Comp

Stability criteriaC1

C2

mmmm1

)0(Li

SDT ST

)( STLi

)(tLIci

cmVin

Vout Vin - Vout

25uA5u x (Vin - Vout)

A = 1

CONTROLTIMING

CRAMP

RAMP

m1

m2

Modulator Gain

8

VIN

ACS

+

-RSN

VSW

The current sense element is usually a resistor or the RDS-ON of the FET.

Output Filter

9

COUT

RESR

RLOAD

VOUTVSW

0.5DmLCf

1RC1ω c

OUTsLOADOUTp1

ESROUTz1 RC

1ω

Control-Loop Considerations Rules of Thumb• Crossover frequency at 1/5th the switching frequency with a

phase margin of 45˚• Higher crossover frequency relates to faster transient response and an

increased likelihood of instability• Lower crossover frequency relates to slower transient response and an

increased likelihood of stability

13

Current Mode Line TransientsPerformance Trade-offs• Sudden changes in the line voltage are alleviated by use of a large

input cap• Inherently better response in current mode because of implicit line

feed-forward• Use of several caps in parallel reduces the ESR also improving

performance• High crossover frequency allows control loop to quickly accommodate

perturbations in the system

15

Current Mode Control Example:LM284x

16

Typical Application CircuitInternal Block Diagram

EMULATED CURRENT MODE (ECM) BUCK REGULATORS

17

Why Emulated Current Mode?

18

Leading edge spike, conventional current mode control.

15W Supply With EmulatedCurrent Mode Regulator

22

6-42V input 5V, 3A output

300 kHz Switching Frequency

23

Thank you!